Lots of good advice here. As others have said, the key is your interaction with the couple. We like to ask questions that help us really get to know a couple -- their likes, dislikes, quirks, what makes them laugh, etc. People like to talk about themselves, so if you show a genuine interest you will have no trouble maintaining conversation.
In addition to what others have said, one thing we have found to be helpful is to provide visuals to help the clients understand what we are looking for. Couple examples:
- Sometimes my wife and I will stand together and "pose" to show the couple what we want to see.
- Occasionally we will show the couple a preview on the back of our camera so they can see for themselves what they look like. Good pictures will boost their confidence, and if they are doing something we don't like, it's easy to point out.
- Describe a scenario to them to help them visualize what you are after. Let's say you want a focused, powerful look from a client -- Just tell them to imagine they are doing a shoot for a magazine cologne/perfume ad. Usually they'll know what those ads look like and will instinctively look at the camera with some intensity in their eyes. They relax because they know what you want, and by telling them a situation it helps some clients to know what you are after without having to describe the exact details.
We have come across some clients who have a hard time relaxing in front of the camera regardless, so there is no magic formula that works every time. Sometimes it just comes natural for clients and sometimes we have to put in extra effort to make a connection and build enough trust in the relationship to where they will let their guard down and relax.
Keep working at it and you should start to see improvement over time.